1. Field of the Invention
This invention relates to sub-sea pipelines carrying petroleum gas or an oil-gas mixture. More particularly, the invention relates to fiber optic sensors for use on sub-sea pipelines as well as methods for deploying the sensors.
2. Description of Related Art
It has been known for many years that fiber optic waveguides can be used to measure temperature. FIG. 1 schematically illustrates a system that employs a fiber optic waveguide to measure temperature. A pulsed-mode high power laser source 1 launches a pulse of light through a directional coupler 3 and along a fiber optic waveguide 2. The fiber optic waveguide 2 forms the temperature sensing element of the system and is deployed where the temperature is to be measured. As the pulse propagates along the fiber optic waveguide 2 its light is scattered through several mechanisms including density and composition fluctuations (Rayleigh scattering) as well as molecular and bulk vibrations (Raman and Brillouin scattering, respectively). Some of this scattered light is retained within the core of the fiber optic waveguide and is guided back towards the source 1. This returning signal is split off by the directional coupler 3 and sent to a receiver 4. In a uniform fiber, the intensity of the returned light shows an exponential decay with time (and reveals the distance the light traveled down the fiber optic waveguide based on the speed of light in the fiber optic waveguide). Variations in such factors as composition and temperature along the length of the fiber optic waveguide 2 show up in deviations from the “perfect” exponential decay of intensity with distance. The receiver 4 typically employs optical filtering 5 that extracts backscatter components from the returning signals. The backscatter components are detected by a detector 6. The detected signals are processed by the signal processing circuitry 7 which typically amplifies the detected signals and then converts (e.g. by a high speed analog-to-digital converter) the resultant signals into digital form. The digital signals may then be analyzed to generate a temperature profile along the length of the fiber optic waveguide 2. This type of temperature sensing is called distributed temperature sensing (DTS) because it measures a temperature profile along the length of a fiber optic waveguide.
Another type of fiber optic sensing is called point sensing. In point sensing, a Bragg grating is etched into a fiber optic waveguide at a desired location. The Bragg grating is designed to reflect light at a particular wavelength. Measurements of wavelength shift of the reflected light can be used to measure temperature or pressure or strain. Multipoint sensors have multiple spaced apart Bragg gratings, which are typically etched to reflect different wavelengths. Analysis of the wavelength shifts of the reflected light can sense conditions at multiple discrete locations along the fiber optic waveguide. Such “point sensing” functionality is described in detail in U.S. Pat. No. 6,097,487, herein incorporated by reference in its entirety.
A typical sub-sea pipeline is composed of a pipe surrounded by one or more layers of protective/insulating material, for example a steel pipe covered with a polymer sheath and then encased in concrete. For fiber optic sensing applications, optical fiber is placed between the pipe and the first layer of protective/insulating material. The sub-sea pipeline is assembled on a barge at sea from sections that are bolted and/or welded together. As sections of pipe are joined together, the ends of the optical fiber for the adjacent pipe sections must be joined to each other. Although such a sub-sea pipeline provides for fiber optic sensing, it suffers from several shortcomings, which include: (i) increased costs and difficulties in integrating the optical fiber as part of the pipeline sections; and ii) increased deployment times and costs as well as maintenance times and costs associated with ensuring the integrity and operation of the fiber optic couplings between section joints of the pipeline.